1. Field of the Invention
The present invention relates to a wireless communication system and a communication method therefor. More particularly, the present invention relates to a wireless communication system for interconnecting an ad-hoc network and an infrastructure network, and a wireless terminal and a communication method therefor.
2. Description of the Related Art
Wireless networks include a cellular communication system in which mobile voice communication and data communication are possible. Conventionally, wireless communication systems in the form of a wireless Local Area Network (LAN), a Metropolitan Area Network (MAN), a Wide Area Network (WAN) and the like are being improved with recent developments of communication technology. An example of a wireless LAN includes a mobile ad-hoc network. The ad-hoc network is a network temporarily constructed by a plurality of wireless terminals without using a fixed network infrastructure. The ad-hoc network is different from general networks in which wired base stations or Access Points (APs) provide service to wireless terminals. In the ad-hoc network, each wireless terminal is a host and can simultaneously function as a base station or router for transferring data to other wireless terminals. Wireless terminals which can be used in the ad-hoc network include various terminals having mobility, such as mobile terminals, notebook computers and Personal Digital Assistants (PDAs), having interfaces for accessing the ad-hoc network. Standardization for the ad-hoc network is currently in the Mobile Ad-hoc Networks (MANET) working group of the Internet Engineering Task Force (IETF).
FIG. 1 is a view schematically illustrating a configuration of a conventional ad-hoc network.
The ad-hoc network is constituted by sub-networks N1, N2 and N3 using multi-hopping routing, wherein wireless terminals are connected to each other through Peer-to-Peer (P2) communication. The wireless terminals 101 to 115 in FIG. 1 can constitute a network by themselves, even without using an existing Infrastructure (IS) network. The IS network may include conventional 2G and 3G networks supporting mobile communication service, and the 4G network currently being developed. In the ad-hoc network illustrated in FIG. 1, the wireless terminals 101 to 115 can access an IS network through a gateway 117, which interconnects heterogeneous networks. In the following illustrations, solid lines drawn between the wireless terminals represent routes through which the wireless terminals can communicate with each other.
In the ad-hoc network illustrated in FIG. 1, a phenomenon is generated where, as the number of routing hops which the wireless terminals 101, 103, 105 and 107 perform for end-to-end data transmission increases, the performance rapidly degrades in terms of transmission capacity, transmission delay and the like. In this case, a problem exists in that a topology change or the like occurs on an intermediate route in the ad-hoc network, and the loss or error probability of transmitted data rapidly increases. Accordingly, research has been conducted on methods for interconnecting an ad-hoc network and an IS network to reduce the multi-hopping probability and to transmit data with just a few hops, rather than using methods for constructing the entire network only with an ad-hoc network.
Differently from an ad-hoc network of a ubiquitous sensor network, an ad-hoc network using wireless terminals must guarantee mobility of wireless terminals and enable the wireless terminals to transmit and receive various contents, such as voice, data and moving pictures, during the movement of the wireless terminals. Therefore, for the ad-hoc network using wireless terminals, a structure of interconnecting an ad-hoc network and an IS network is suitable, rather than a pure ad-hoc network structure, such as a sensor network. In the following description, the term “ad-hoc network” denotes an ad-hoc network using wireless terminals.
In the case where an ad-hoc network and an IS network are interconnected, an Industrial, Scientific, and Medical (ISM) band is used in the ad-hoc network, while a licensed frequency band other than the ISM band is used in the IS network. The ISM band is a frequency band which is reserved for use by industrial, scientific, and medial fields all over the world. Considering the structure of interconnecting an ad-hoc network and an IS network, a wireless terminal must include two interfaces for accessing two mutually different frequency bands. Therefore, the construction of the wireless terminal is complicated and implementing technology for cooperating frequency bands between the ad-hoc network and the IS network is difficult.
Meanwhile, one principal effect of the ad-hoc network is that wireless terminals can communicate with each other in shadow areas, where access devices, such as base stations and APs, do not support communication. An alternative for solving the shadow area problem is a relay scheme for coverage expansion, which is proposed in the Institute of Electrical and Electronics Engineers (IEEE) 802.16j standard.
FIG. 2 is a view schematically illustrating a configuration of a conventional network system to which a relay scheme for coverage expansion is applied.
An example of the relay scheme applied to the network system illustrated in FIG. 2 is a Mobile Multi-hop Relay (MMR) scheme defined in IEEE 802.16j.
According to the relay scheme of FIG. 2, relay stations 203 and 205 are installed at areas outside the radius of cell C0 in order to support the communication of wireless terminals which exist outside the radio coverage of an AP 201, such as a base station. The relay stations 203 and 205 relay signals transmitted and received between the AP 201 and wireless terminals 207 and 209 in order to support communication of the wireless terminals 207 and 209, which are located outside the cell. For example, in the case of a relay station employing an MMR scheme proposed in IEEE 802.16j, the two relay stations can be connected in a mesh communication scheme, and each wireless terminal located outside the cell is connected to the nearest relay station.
The relay scheme of FIG. 2 has an advantage in that shadow areas are eliminated. However, the relay scheme of FIG. 2 has disadvantages in that coverage expansion is limited because the maximum number of hops for signal transference is limited to three and the relay stations are poor in flexibility.
When a multi-hop transmission is performed using an ad-hoc network, as described above, the shadow area problem can be solved. However, many problems, such as transmission delay, error rate increase, degradation of transmission bandwidth and frequent change of the transmission path occur in implementing the ad-hoc network. In addition, when a frequency in the ISM band is used in the ad-hoc network, frequency interference may occur with other communication schemes using the ISM band. Also, when an ad-hoc network and an IS network are interconnected, the implementation technology is complicated, as described above, and the service provision range is limited because authentication of terminals cannot be performed due to characteristics of the ad-hoc network using the ISM band.
Therefore, a need exists for a wireless communication system and method for interconnecting an ad-hoc network and an IS network.